1. Field of the Invention
This invention relates to a coupling for a two-part valve stem used in fire hydrants and a method for making the coupling. More particularly, this invention relates to improving the durability and corrosion resistance of the coupling and its securing pins.
2. Prior Art Valve Stem Couplings
A fire hydrant's water valve is buried deep underground. In order to allow this valve to be operated from the surface, a long valve stem extends from the valve up to the surface.
When an above ground portion of a fire hydrant is knocked over, broken, or dislodged, the valve stem can be ripped from the valve, twisted, bent, or otherwise damaged. Furthermore, the force on the valve stem may cause damage to the water valve itself. Therefore, in order to minimize damage to the valve and to facilitate inexpensive and easy repair after such an accident, fire hydrants typically employ two-part valve stems.
U.S. Pat. No. 3,439,947 issued to Luckenbill et al shows one type of such a two-part valve stem. The valve stem comprises a lower valve stem and an upper valve stem. The lower valve stem leads from the water valve up to a level just beneath the surface of the ground. The upper part of the valve stem leads from the top of the lower valve stem up into the fire hydrant above ground. These two parts of the valve stem are connected via a coupling which is designed to yield or tear before the lower valve stem and/or water valve can be damaged.
As is shown in U.S. Pat. No. 3,439,947, such a coupling is usually a cylindrical shaped collar which simultaneously fits over the top end of the lower valve stem and the bottom end of the upper valve stem. The coupling is attached to both parts of the valve stem by two transverse pairs of aligned apertures. The upper pair of apertures and an accompanying upper securing pin connect the coupling to the upper valve stem. The lower pair of apertures and a lower securing pin connect the coupling to the lower portion of the valve stem.
It has been recognized that if the coupling fails at the lower apertures, the coupling will break away from the lower valve stem. Replacement of the upper valve stem is then considerably easier because the coupling is easily removed from the hydrant because it is attached to the upper part of the valve stem. After the coupling is torn from the lower securing pin, the lower securing pin is left fitting loosely in the lower valve stem. The lower securing pin is then also easily removed.
In order to ensure that the coupling fails before the valve stem or the securing pins, the coupling is made of a low ductility material such as the higher tensile strength versions of low carbon steel. The securing pins, on the other hand, are made of a comparatively tougher material such as stainless steel. The coupling will therefore rip or tear before the securing pins.
To ensure that the coupling will rip from the lower securing pin and not fail elsewhere first, the portion of the coupling surrounding the lower apertures is made especially weak. For example, a notch is provided in the bottom end of the coupling so that less coupling steel exists between the securing pin and the bottom of the coupling. The lower securing pin should normally rip from the coupling before the coupling fails elsewhere.
In the field, such couplings often encounter highly corrosive water. Because the couplings are made of low carbon steel, they corrode quickly when exposed. The steel of the couplings are therefore often protected with a corrosion resistant plating of zinc or cadmium. Such presently known couplings are widely used in fire hydrants and are widely believed to function well in common hydrant environments.